Viga buque: Análisis de vibraciones forzadas por excitaciones de la hélice. Hull girder: Forced vibration analysis by propeller excitations

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1 Viga buque: Análisis de vibraciones forzadas por excitaciones de la hélice Hull girder: Forced vibration analysis by propeller excitations Franklin Jhonny Domínguez Ruiz Tecnavin S.A. Guayaquil, Ecuador Telephone 593(9) ABSTRACT The non-uniform wake around the propeller generates fluctuating forces on the propulsion shaft. This article presents a methodology used for the forced vibrations analysis of hull girder due to this propeller excitation. This approach is applied to a research boat considering the propeller working in the operating range using a finite element model including all ship structures, rudder, and propulsion lines with their respective supports. Added mass and damping in all submerged elements were also considered. Vibration levels acting in the vessel structure are compared with the limits proposed by ISO 6954 (2000). Keywords: lateral vibration, finite element model, RESUMEN La estela no uniforme alrededor de la hélice genera fuerzas fluctuantes en el eje de propulsión. Este artículo presenta una metodología usada para el análisis forzado de vibración de la viga buque debida a esta excitación de la hélice. Este procedimiento es aplicado a una lancha de investigación usando el método de elementos finitos incluyendo todas las estructuras de la nave, timón y líneas de propulsión con sus respectivos apoyos, considerando la hélice en el rango de operación. La masa añadida y amortiguamiento de todos los elementos sumergidos también se consideran en el análisis. Los niveles de vibración obtenidos en la estructura de la embarcación se comparan con los límites propuestos por ISO 6954 (2000). Palabras claves: vibración lateral, modelo de elementos finitos, modos de vibración de viga buque 1. Introduction Dynamic propeller forces need to be included to accurately verify that the hull girder supports all loads acting on it. These forces are a function of thrust, torque, and propeller frequency. Namely, it is a function of: i) rotational speed for fixed pitch propellers, and ii) pitch angle for controllable pitch propellers. In the present work, Finite Element Method was used to estimate the deformation due to these forces acting over the hull girder. This method allows modeling hull girder considering all structural elements and the propeller dynamic forces. Numerical results expressed in RMS speed of vibration are compared to the limits proposed by the Classification Societies. 2. Finite Element Method Applied to the Hull Girder There are several recommendations for the development of a Finite Element Model (FEM), especially by Classification Societies, in the present work; it was necessary to include mass and inertia of the structural elements of hull and superstructure, incorporating machinery foundations for hull girder vibration analysis. Shell elements were used in the hull and in primary structures, and frame elements to model the secondary structures and pillars, see Figures 1 and 2. The mass of the structure, equipment, tank liquid, and the added mass values of 40 and 20 [kg/m2] on decks and sides respectively were distributed on the corresponding nodes following Germanischer Lloyd (GL) recommendation.

2 Figure 1 Ship Finite Element Model Figure 2 FEM interior view: Engine room 3. Hydroelastic Hull Behavior Restraints were placed to simulate the ship behavior into the water, using an equivalent spring system placed in the submerged surface of the hull to balance the ship weight. Spring stiffness was calculated using the volume of water displaced within specific sections, see Figure 3. To verify that the restrictions are conveniently applied, each node deformation was verified using static weight of the vessel. The added mass can be obtained from Seakeeping analysis for each speed and sea state [Lewis, FM 1929]. This mass is a function of the vessel encounter frequency,. cos (1) Where ωe= Encounter frequency (rad/s), ω= Wave frequency (rad/s), g= Gravity acceleration (m/s2), U= Ship speed (m/s), and µ= Wave incidence angle (rad) Added mass values for three different ship speeds with the same sea state 3 and following seas are shown in Table 1. Table 1 Hull added mass Sea Added % of Vel. ωe State mass displacement Knts rad/s ton % % % % 5. Propulsion line behavior The propulsion line transmits the thrust of the ship as well as the exciting forces from the propeller. It s lateral, axial, and torsional natural frequencies need to be considered to assess resonance. Figures 4 and 5 show the FEM propulsion line included in this analysis. Hull Propeller Bearings Propulsion shaft Gearbox bearing Figure 4 Longitudinal cut at propulsion line Figure 3 - Aft frame Hull bottom restraints 4. Hull added mass estimation On elements that are submerged in water vibration moves a small fluid volume; its mass is called added mass. This mass is added to FEM model as a distributed mass over all submerged elements. Rudder Propeller Bearings Hull Figure 5 Propulsion line in FEM 5.1. Bearings location Cutless bearings or roller bearings are included in our FEM model. Usually, bearing center is the support point, except for bearing close to the propeller, which is considered to 1/3 from the aft end of bearing.

3 The propulsion line natural frequencies depend on bearing position and stiffness. In the present study, 3 bearings were used, as shown in Figure 6. Two aft supports are bronze rubber cutless bearings. The propeller shaft had been modeled using beam elements. The propeller and flanges masses are included in their respective locations. The manufacturer provided the bearing stiffness value for accurate results. Figure 6 Propulsion line drawing 5.2. Propeller added mass The propeller accelerates its surrounding water and an added mass is generated, that was estimated using PRAMAD program [U OF MICHIGAN, 1980]. There are several formulas for estimating these masses M. Parsons (1980), Schwanecke (1963) or D. MacPherson (2007) Table 2 Propeller added mass Direction Units Value M11 N.s2/m M22 N.s2/m 48.9 M52 N.s M55 N.m.s Stuffing box Thrust bearing Gearbox mounts Engine mounts Figure 8 Thrust bearing and engine/gearbox mounts Natural vibration analysis Once the mentioned hull and propulsion line properties are modeled, vibration analysis can be performed for both, hull girder and propulsion line natural frequencies. The FEM and the eigenvalue matrix method had been used to calculate the propulsion system vibrational modes. The finite element method divides a body in finite elements interconnected by nodes, which are equivalent to the original body; in the elastic zone the equations to find the nodes deformation can be expressed in matrix form as follows: 0 (2) where: [M] is the mass matrix of the system [K] is the stiffness matrix of the system is the displacement vector is the second derivative of displacement Y Nowadays, computers allow this calculation accurately and for several degrees of freedom. Tables 3 and 4 present the natural frequencies in the system working range. Figure 7 Added mass nomenclature 5.3. Thrust bearing and engine/gearbox flexible coupling Stiffness In our model, thrust bearings are placed on the gearbox; and flexible mounts of gearbox & engine with the stiffness provided by manufacturer. Mounts properties were modeled for each direction (x, y, z). Figure 8 shows the position of the elements used. Table 3 Hull girder natural frequencies Vertical direction Mode 1 Mode 2 Mode 3 Mode 4 Hz Hz Hz Hz Horizontal direction Mode 1 Mode 2 Mode 3 Mode 4 Hz Hz Hz Hz Table 4 Propulsion line natural frequencies Mode 1 Mode 2 Hz Hz

4 Figures 9 and 10 show the modal shape of natural frequencies It is recommended that the working range has to be from 650 RPM to 2000 RPM on engine, due to coincidence between hull girder natural frequencies and engine and propeller excitation range as can be seen in Table 5. Figure 11 First vibration mode Hz Figure 12 Second vibration mode Hz Propulsion line natural frequencies are within the working range and forced analysis should be considered to check the structures resistance and whether the proposed vibration levels standards are met. Figure 9 Vertical direction mode shapes Damping Energy due to vibration on the ship can be dissipated as damping. Vibration analysis should consider three types of damping, namely: the propeller damping, the hysteresis damping and hull damping in water Propeller damping Damping is generated when the propeller rotates in the water, the approximation of these values are shown in Schwanecke (1963) or M. Parsons (1980). The damping depends on the propeller rotation speed, therefore is determined for each operating condition, see Table 6. The damping is placed on the propeller node in the FEM. Structural deformations caused by the propeller excitation forces decreases due to the damping effect. Figure 10 Horizontal direction mode shapes Table 5 - Engine and propeller working ranges Engine working range RPM Hz Propeller working range RPM Hz Figures 11 and 12 show the natural frequencies mode shape found in the propulsion line. It should be noted that the first vibration mode is at the tunnel between Stern tube bearing and gearbox and the second mode at the propeller end. The first axial natural frequency is Hz. Table 6 - Propeller damping for vibration analysis Damping (Schwanecke) Engine RPM Frec HZ C11 N.s/m C22 N.s/m C52 N.s C55 N.m.s Structure damping Hysteresis damping is caused by internal molecular friction on vessel structures, and its value is estimated using a coefficient 0.05 proportional to the stiffness.

5 Hull damping Ship motion generates a damping (B 33 ) which can be obtained from Seakeeping for each speed and sea state under study. These results are applied to the submerged hull Transmissibility Flexible mounts reduce vibration effect produced by the engine on their foundations. In the case of study, the propulsion system has 2 front flexible rubber mounts for each engine and 2 flexible mounts for each gearbox. k 2 F c k 2 Figure 13 Perturbing force transmitted by the springs and damper. Thomson W., (1972) Transmissibility is the relationship between the perturbing force and the transmitted force to the foundation and depends especially of the connection stiffness between the engine and the boat structure. For this study, flexible rubber mounts had been used. There are several references that provide recommendations to know whether a particular mount is suitable to reduce engine forces transmission to structures. W. Thomson, (1972), shows a graph that has frequency (cpm) and the static deformation produced by the engine on the flexible mount or the connecting element as variables. For the present study, the engine manufacturer provides several options for flexible mounts, tables 7 and 8 show the percentage of effectiveness of the system foundation. According to transmissibility analysis results had been decided to use the 315 mount - 55SH due to the appropriate reduction of excitation forces transmitted by the engine and therefore these forces will not be considered in the analysis of vibration of the boat. Table 7 - Analysis of transmissibility for engine mount at 650 RPM 650 RPM Equiv. arrangement Effectiveness % Flexible mounts db Isolation RD314 B-65Sh % RD314 B-60Sh % RD314 B-55Sh % RD314 B-50Sh % RD314 B-45Sh % RD315 HD-65Sh % RD315 HD-60Sh % RD315 HD-55Sh % RD315 HD-50Sh % RD315 HD-45Sh % Table 7 - Analysis of transmissibility for engine mount at 2000 RPM 2000 RPM Equiv. arrangement Effectiveness % Flexible mounts db Isolation RD314 B-65Sh % RD314 B-60Sh % RD314 B-55Sh % RD314 B-50Sh % RD314 B-45Sh % RD315 HD-65Sh % RD315 HD-60Sh % RD315 HD-55Sh % RD315 HD-50Sh % RD315 HD-45Sh % 7. - Forces and moments of propeller excitation Propeller vibration forces are predominant in calculating propulsion line and boat structure vibration. These forces occur due to non-uniform water flow in the propeller creating periodic forces depending on the number of blades called propeller excitation forces. These forces are generated in the vertical, transverse and longitudinal directions.

6 Forces transmitted to the propulsion shaft (bearing forces) For lateral vibration analysis (bending) should be considered vertical forces F 33 and transverse F 22 and their moments M 33 and M 22, while for the axial analysis the longitudinal force F 11 is considered, following same nomenclature shown in Figure 7. Exciting forces are decomposed into harmonic components using the Fourier analysis [Kumai, 1961]. Currently, Classification Societies recommend excitation values for each order based on the number of blades and thrust or torque on the propeller, as appropriate. For the study boat, the excitation values recommended by ABS (2006) had been used. The values of the forces applied to the study boat in 4 different working conditions are shown in Table 9. Table 9 - Propeller excitation forces for vibration analysis Frequency (HZ) Order 1Z 1Z 1Z 1Z Engine RPM Total thrust Total torque Propeller excitation forces Axial F11 (N) Vertical F33 (N) ,099 Transv. F22 (N) Moment M11 (N.m) Moment M33 (N.m) Moment M22 (N.m) Hull pressure forces (pressure fluctuation) There are several causes that produce fluctuating pressures on the hull in the area of the propeller. These pressures fluctuate proportional to the propeller rotation speed, its number of blades (blade rate frequency), and cavitation. Pressures can be obtained by experimentation, by numerical approximation (CFD) or by empirical formulas [Holden, 1980]. For the current analysis, Holden formulas were used. These pressures vary according to working condition. Table 10 shows pressures values applied in an area of 1 m2 of each propeller, in all working conditions analyzed. Table 10 Hull fluctuating pressures applied Frec. (HZ) Order 1Z 1Z 1Z 1Z 1Z 1Z Engine RPM Pressure PT (N/m2) Working conditions to evaluate For this study case, the reduction ratio is R = 3.04, and a 4 blades propeller was used. Therefore, the excitation will occur at a frequency: (3) Generally, the two first orders of the propeller excitation are considered: 1Z and 2Z, due to lower excitation magnitudes presented by higher orders. Resonance conditions between the excitation frequency and propulsion line natural frequencies had been analyzed. Additionally maximum working condition (MCR) had been analyzed, which in this case is 2000 RPM. The following table shows the resonances to consider Table 11 Resonances table Natural Frec. (HZ) Order 1Z 1Z 1Z 1Z 1Z 1Z Engine RPM Rudder line behavior Vibration analysis must consider the rudder behavior, to check if there is any resonance in the working range. Additionally it is important to know if the rudder holds up propeller fluctuating stress loads. Rudder shaft Supports Rudder Figure 14 Rudder finite element model

7 Rudder supports location Rudder supports are usually cutless bearings. The FEM represents these supports in the corresponding directions. In the present study, rudder has two supports, upper one restricting rudder shaft axial movement and allowing only rotation and lower support. Due to the rudder shaft is modeled with frame element; constraints simulating the contact between the flanged shaft and the rudder shell had been included Rudder added mass Rudder is also immersed in water and its added mass is considered in the FEM. Rudder is considered as a plate to find the rudder added mass. Mukundan (2002) method was used and its values are presented in Table 12. The rudder added mass was evenly distributed at nodes on rudder surface in their respective directions. Table 12 Calculated rudder added mass Transversal added mass C 0.73 psw 1025 Kg/m 3 B m L 1.44 m B/L M* y Kg Longitudinal added mass C 0.21 psw 1025 Kg/m 3 B m A m B/L 4.53 M* x Kg Added inertia C 0.73 psw 1025 Kg/m 3 B m L 1.44 m B/L MI* 12.2 Kg.m Classification Societies Acceptance criteria Classification Societies recommend limits for vibration velocity for crew, passengers, structures and machinery areas. These limits are recommended to ensure people comfort in the accommodation areas and to prevent fatigue failure in local structures Living areas limits Classification Societies recommend limits depending on craft type and accommodation or work sectors. ISO 6954 (2000) proposed by Classification Societies such as ABS and Germanischer Lloyd had been taken as acceptance criteria for the evaluation of the study boat. On Table 13, the classification refers to the area of application: A Class: is for passenger cabins, B Class for accommodation of crews and C Class for workspaces Table 13 - RMS vibration limits from ISO 6954 (2000) of 1-80 Hz RMS values of global vibration Classification A B C mm/s mm/s mm/s Values on which adverse comments are probable Values below which adverse comments are not probable There are voluntary limits known as comfort notations, which are limit values proposed by the Classification Societies to grant class notations, especially for passenger vessels Structure limits There are vibration limits for not accommodating areas as tanks, mast, lazaretto structures, engine room, etc. These limits seek to avoid structural damage due to fatigue and the cracks occurrence due to vibration. Figure 15, taken from ABS (2006), shows vibration peak limits for structures bellow which the risk to fatigue crack is expected to below. From Figure 15 can be seen that for frequencies between 5 Hz and 10 Hz vibration peak limit recommended is 30 mm/s 10. -Hull structure and deck house forced analysis evaluation Finite element method was used to perform a forced vibration analysis of the hull structure, using the following equation: (4)

8 where [M] is the mass matrix of the system, [K] is the stiffness matrix of the system, [C] is the damping matrix system, {Y}: is the displacement vector, {Ẏ}the first derivative of displacement Y, is is the second derivative of displacement Y, {F}: is the excitation force vector Figure 16 Deformation (mm) at Hz condition Figure 17 Deformation (mm) at 29.73Hz condition Figure 18 Deformation (mm) at Hz condition1 Figure 15 Peak vibration limits for local structures Deformation was estimated at all nodes in the model, for each load condition. Figures 16, 17, 18 and 19 graphically show deformation results. Harmonic motion deformation as the case of vibration can be represented as follows, at any time t: sin. (5) where: A = deformation amplitude (m) = Vibration frequency (rad/s) t = time (s) Since the speed is the relationship between the deformation and the time, the vibration speed magnitude (V) can be obtained by the following equation: (6) Figures 16, 17, 18 and 19 show as color-sectors the deformation that is proportional to vibration speed. There is greater deformation on aft bulkhead of upper deck house at Hz condition. Figure 19 Deformation (mm) at Hz condition Tables 14 and 15 show vibration speed values by sector for each condition, calculated from the deformation found with the FEM. Most vibration levels do not exceed the limits set by the rules, except the aft bulkhead on upper deck at 2000 RPM, where the limit is 6 mm/s Local Reinforcements to reduce higher vibration levels The FEM allow us to carry out structural modifications to comply with the recommended limits. In this case, it is requested to increase the section of the vertical reinforcements to the aft bulkhead on upper deck house. Figure 20 shows the deformation in the same scale as the previous figures and shows the deformation decrease on aft bulkhead at upper deck, with respect to Figure 19. This improvement can be seen in Table 16.

9 Table 14 - Vibration speed at accommodation RMS vibration velocity(mm/s) Frec, (HZ) Sector ACCOMODATION AREAS Inner main deck Fore exterior main deck Exterior deck Interior deck Upper side deckhouse Lower side deckhouse Aft bulkhead at deck Table 15 Vibration speed at structure RMS vibration velocity(mm/s) RPM Frec. (HZ) Sector STRUCTURE Long. Beam over strut Hull at stuffing box Long. Beam over tunnel bearing Pilot house roof Transom/ side intersect Figure 20 Deformation (mm) at Hz condition (reinforced bulkhead at upper deck) Table 16 Vibration speed after upper deck bulkhead reinforcement RMS vibration velocity(mm/s) RPM 2000 Frequency (HZ) Sector Limit Direction ACCOMODATION AREAS Aft bulkhead at deck X Conclusions Acceptance criteria are effective, so the best way to avoid resonance problems is configuring the propulsion system to keep vibration below criteria. Natural frequencies of the propulsion line need to avoid the working range to prevent resonances. Natural frequencies of ship panels and structure need to avoid the working range of propeller excitation forces. Forced vibration analyses on hull girder including propeller excitation forces should be performed to identify sectors that do not meet standards. The results obtained in the design stage allow identifying possible failures, especially when there is resonance risk in the propulsion line. 13. Bibliography AMERICAN BUREAU OF SHIPPING, Guidance on ship vibration, Houston, ABS, 2006 DEPT. OF NAVAL ARCHITECTURE AND MARINE ENGINEERING OF UNIVERSTIY OF MICHIGAN, Propeller added mas and damping program (PRAMAD). Revision of September 5, 1980.

10 H. MUKUNDAN, Finite Element Analysis of a Rudder, Undergraduate Thesis for the completion of Bachelor of Technology in Ocean Engineering and Naval Architecture, Indian Institute of Technology, Madras, July 2002 HOLDEN, FAGERJORD, FROSTAD, Early Design stage approach to reducing hull surface force due to propeller cavitation, SNAME Trans., 1980 ISO, International Standard ISO 694: Mechanical vibration Guidelines for the measurement, reporting and evaluation of vibration with regard to habitability on passenger and merchant ships, 2000 IWER ASMUSSEN, WOLFGANG MENZEL, HOLGER MUMM, Ship vibration, Hamburg, Germanischer Lloyd, 2001 LEWIS F.M., The inertia of the Water Surrounding a Vibrating Ship, SNAME transactions,1929 M. PARSONS, Added mass and damping of vibrating propellers, Department of naval architecture and naval engineering University of Michigan, 1980 RAMESWAR BHATTACHARYYA, Dynamics of marine vehicles, John Wiley & Sons, Inc., 1978 Engine data Strokes 4 Cylinders Nº 12 V angle 90 GRADES Bore 165 mm Stroke 190 mm Connecting rod l. 354 mm Weight 6800 Kg Minimum rpm 500 Maximum rpm 2000 Max power x rpm 1680KW x 2000 RPM 1680 kw 8.02 KN.m Propeller data Type Fixed pitch Blades Nº 4 Diameter m Pitch m D.A.R 0.91 Mass Kg Polar inertia Kg.m2 Gearbox data Ratio 3.04 S. QUEK, G.R. LIU, Finite Element Method: A Practical Course, Burlington MA, Elseiver Science, 2003 SCHWANECKE H.,Gedanken zur Frage der hydrodynamisch erregten Schwingungen des Propellers und der Wellenleitung, Jahrbuch STG, 1963 T KUMAI, Some Aspects to the Propeller Bearing Forces Exciting Hull Vibration of a Single Screw Ship, Research Inst. for Applied Mechanics, Kyushu Univ, 1961 THOMSON W., Theory of vibration, New Jersey, Prentice Hall, Annex Boat main particulars Ship type Investigation boat Overall length: 46 m Bea: 7 m Depth: 4m Draft: 1,9 m